Nearly let loose for ever (9)
I believe the answer is:
eternally
'for ever' is the definition.
(I've seen this before)
'nearly let loose' is the wordplay.
'loose' indicates an anagram (letters are loosened from their original positions).
'nearly'+'let'='nearlylet'
'nearlylet' is an anagram of 'ETERNALLY'.
(Other definitions for eternally that I've seen before include "Always and ever" , "For evermore" , "Always and forever" , "constantly" , "all the time" .)